The rotational orientation of an intraocular lens (IOL) is becoming an increasingly important aspect of modern cataract surgery. While sophisticated algorithms involving automated eye tracking have evolved for monitoring the pupil center and corneal limbus during laser vision correction surgery, each of these require the ability of the eye to actively fixate on a target. This approach may not be available in procedures such as cataract surgery where an anesthetic is typically used, such as a periocular anesthetic injection rendering the eye unable to voluntarily focus or fix on a target. Even when topical anesthesia is utilized, it can be difficult for a patient to maintain fixation and the physiologic pupil cannot be used, as the pupil must remain dilated while the IOL is inserted and rotated into position.
A laser vision correction treatment can be positioned to center over the physiologic pupil. However, for an intraocular lens centered inside the capsular bag of the crystalline lens (which has been removed) or the ciliary sulcus, the center point of the IOL is more closely related to the corneal vertex than the pupil center, because the anatomy of the capsular bag and the ciliary sulcus more closely corresponds to the corneal vertex. If the rotational axis of an IOL is to be based on the pupil center, alignment must be determined with reference to the dilated pupil because the center of an undilated pupil (the pupil centroid) will vary up to 1 mm in a normal eye with constriction or dilation in relation to photic or accommodative stimuli. In contrast, the center of a dilated pupil remains more constant.